In the description of technical specification 36300-910 of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), the Long Term Evolution (LTE) network consists of the E-UTRAN Macro Evolved NodeB (Macro eNB), the HeNB, and the Evolved Packet Core (EPC). The HeNB supports three modes for accessing of a user: Closed Subscriber Group (CSG), Hybrid and Open. E-UTRAN comprises the collection of the Macro eNB which is connected with EPC via an S1 interface and the HeNB; the Macro eNBs can be connected via an X2 interface, and the X2 interface may be defined between the HeNBs or between the HeNB and the Macro eNB. S1 and X2 are logic interfaces. One EPC can manage one or more eNBs; one eNB can be controlled by multiple EPCs, and one eNB can manage one or more cells.
Self Organizing Network (SON) is a technology for automatically implementing network configuration and optimization. The characteristic of the technology is automatic configuration and automatic optimization. By applying this technology in LTE, an eNodeB can automatically configure network parameters, and automatically optimize according to a network change, so as to improve network performances and save a great number of manpower and material resources at the same time.
One researching point of SON is Mobility Robustness Optimization (MRO), which is for solving problems of too late handover, too early handover, or handing over to a wrong cell which may be generated when a User Equipment (UE) implements a handover among cells. Too late handover refers to that when a UE triggers a handover or the handover is still not triggered, received signal strength of a source cell is very weak, and thus the UE generates a Radio Link Failure (RLF), wherein the characteristic of too late handover is that the UE has a link failure with the source cell and the UE establishes a connection with another cell after the link failure is generated. Too early handover refers to that a handover is implemented too early and a link failure is generated, wherein the characteristic of too early handover is that the UE establishes a connection with a target cell successfully, the UE has a link failure immediately, and then the UE establishes a connection with the source cell. Handing over to a wrong cell refers to that an incorrect target cell is selected during handover, wherein the characteristic of handing over to a wrong cell is that the UE establishes a connection with a target cell successfully, the UE has a link failure immediately, and then the UE establishes the connection with a third party cell which is neither the source cell nor the target cell.
After the UE re-establishes a connection with a new cell, the eNodeB controlling the new cell sends a RLF Indication message to the eNodeB controlling the source cell, so as to enable the eNodeB controlling the source cell realizing the detection to the above mentioned MRO. At present, the RLF Indication message comprises the following contents: a Physical Cell Identity (PCI) of a cell where the RLF is sent, an E-UTRAN Cell Global Identifier (ECGI) for re-establishing connection with a cell, a Cell Radio Network Temporary Identifier (C-RNTI) of the UE in the source cell, and a short Medium Access Control Integrity protection (shortMAC-I), wherein the shortMAC-I is optional. According to the received RLF Indication, the eNodeB controlling the source cell determines whether the handover is too late or too early, or whether a wrong cell is selected, and notifies an eNodeB re-establishing a connection, so as to optimize handover parameters between cells and improve Robustness of handover on the above basis.
For a handover to a target cell which is CSG or Hybrid (the UE accesses in with a CSG mode, generally named as CSG cell below), the measurement configuration before the handover is different from a handover of which the target cell is a Macro NodeB, the UE needs to trigger a measurement for the CSG cell according to its own fingerprint matching result. For hetero-frequency conditions, the UE can use a Proximity Indication (PI) message to notify the current serving cell to implement corresponding measurement configuration, wherein the PI message comprises an indication of entering into/leaving a range of the CSG cell, and frequency information of the CSG cell. FIG. 1 is a diagram of a RLF scene. As shown in FIG. 1, under such a circumstance, if the UE cannot send the PI message in time, or the PI message is lost, or the frequency information contained in the PI message is different from the frequency used by the CSG cell, the UE cannot be handed over to the corresponding CSG cell in time. As the CSG cell is indoor, when the user enters into the room, the signal of the current serving eNodeB may be too weak to maintain the communication of the UE, thus the above mentioned problem of too late handover can arise. Under such a circumstance, if implementing a normal processing method for the too late handover, unnecessary adjustment for the handover parameters is needed, thereby other problems of handover may be caused. Therefore, E-UTRAN needs to distinguish the specific reasons for generating a too late handover.